The concept that renin is a single substance and that the plasma renin activity is controlled only by the secretory mechanism is now being replaced by a more comprehensive concept that various forms of activatable renin or renin precursors exist and changes in their molecular structures also play important roles in the regulation of renin activity. However, our knowledge on the nature of the renin precursors and on their physiological significance is meager and is in a state of confusion. We propose to provide answers to these problems by studying the molecular structure of renin and renin precursors and by elucidating the mechanism of their activation. Lack of pure preparations of renin in the past prevented studies of regulatory mechanism of renin on the molecular level. Recent advent of the purification technique of this enzyme, and the availability of a large quantity of renin from the mouse submaxillary gland now permit studies on structural features which contribute to the unique function of renin in blood pressure regulation. We will investigate the structural features which play vital roles in the renin action and its regulation. Various forms of renin including the activatable form and/or renin precursors will be identified unequivocally by their structural features. Mechanism of the conversion or activation of the precursors to their active form will be clarified again in terms of structural change. Preparatory steps to the elucidation of three-dimensional structure of renin and its precursors is causing a serious confusion in our understanding of the molecular mechanism of blood pressure regulation, the present studies are expected to make important contributions to many of the puzzling problems and improve our knowledge on the mechanism of the regulation of renin activity, thus permitting more rational approach to the diagnosis and treatment of hypertension.